1. Field of the Invention
The invention relates to a method for setting an optimum value of a write parameter for use in an optical recording apparatus for writing information on an optical recording medium by means of a radiation beam, the method comprising writing a series of test patterns on the recording medium, each pattern being written with a different value of a write power level (P) of the radiation beam, reading the patterns so as to form corresponding read signal portions, and deriving a value of a read parameter from each read signal portion. The invention also relates to a method for setting an optimum value of the write power level of the radiation beam.
The invention also relates to an optical recording apparatus for recording information on an optical recording medium, said optical recording apparatus comprising a radiation source for emitting a radiation beam having a controllable value of a write power level for recording information on the recording medium, a control unit for recording a series of test patterns, each pattern being recorded with a different value of the write power level, a read unit for reading the patterns and forming corresponding read signal portions, and first means for deriving a value of a read parameter from each read signal portion.
The invention also relates to an optical recording medium onto which information may be recorded by irradiating the recording medium with a radiation beam, the recording medium comprising an area containing control information indicative of a recording process whereby information can be recorded on said recording medium, the control information comprising values of recording parameters for the recording process.
2. Description of the Related Art
A method and apparatus according to the first paragraph are known from the European Patent Application No. EP 0 737 962, corresponding to U.S. Pat. Nos. 5,740,149 and 5,761,179. The apparatus uses a method which includes the following steps for setting the optimum write power (Popt) of the radiation beam. First, the apparatus records a series of test patterns on the recording medium, each pattern with increasing write power (P). Next, the apparatus derives the modulation (M) of each pattern from the read signal corresponding to the pattern. The apparatus calculates the derivative of the modulation (M) as a function of the write power (P) and normalizes the derivative by multiplying the derivative by the write power (P) over the modulation (M). The intersection of the normalized derivative (γ) with a preset value (γtarget) determines a target write power level (Ptarget). Finally, the target write power (Ptarget) is multiplied by a parameter (ρ) so as to obtain a write power level (Popt) suitable for recording on the recording medium. The value of the parameter (ρ) is read from the recording medium itself. The test patterns are recorded on the recording medium by applying write power (P) values in a range around a given value (Pind) which is also read from the recording medium itself.
In an optical recording apparatus, it is important to record information on optical recording media with the correct power of the laser beam. A media manufacturer cannot give this correct power in an absolute way (for example, pre-recorded on the disc) because of environment and apparatus-to-apparatus deviations for every recording medium and recording apparatus combination. The known method for setting the optimum write power (Popt) takes the different characteristics of the recording media into account by measuring the modulation (M) of the test patterns written on the recording media. Furthermore, this method is independent of the specific recording apparatus. The method is designed for providing a proper setting of the write power for each combination of recording apparatus and recording medium.
However, it is a disadvantage of the known method in that it is not always possible to determine an accurate and unambiguous value for the target write power level (Ptarget) and, therefore, for the optimum value (Popt) of the write power level (P). This is because of the measurement noise introduced during the measurement of the values for the modulation (M) of each pattern. This measurement noise increases with decreasing write power (P) of the test patterns. It appears that even when the measured modulation values are averaged so as to reduce the measurement noise, sometimes a sort of plateau occurs in the γ-curve, thus preventing the determination of an unambiguous value for the target write power level (Ptarget).